The content entry experience is broken if the layout paragraph template is overriden in the theme.
I have a paragraph called section, which has the layout behavior option. This has a template called paragraph--section--default.html.twig
The template loads when viewing content or when the edit page is opened for the first time. However, if I edit the layout settings like color o bg image, the template is no longer being used and instead loads the template from modules/contrib/paragraphs/templates/paragraph.html.twig
Make sure the current conten theme is used when rendering the layout paragraph contents.
Active
2.2
Code
As a fix on my project, I created a Config Override service that checks if the active route belongs to Mercury Editor as in an AJAX request.
If it does I override system.theme.default with my theme. Ideally at this point I would trigger any theme switcher rules, but for the moment nothing complex is required so I force the theme I need.